One type of surgical handpiece used in cataract surgery uses water-jet based liquefaction devices that generate pulses of heated surgical solution, which delaminate and separate lens tissue. The surgical solution or injection fluid is typically a sterile heat balanced salt solution. Pulses of injection fluid are delivered through a smooth polymer tip into the eye. The surgeon controls the pulses of injection fluid using controls that are typically provided on a console, the handpiece, or a foot pedal. For example, the surgeon can control the pulse strength by altering volume and velocity, varying the pulse rate, activating burst modes that use variable rest intervals, and/or decreasing repulsion of nuclear material.
One of the keys to controlling the injection fluid pulses is controlling the pneumatic pressure of the container that supplies the injection fluid to the handpiece since the pressure of the container is a key factor that determines the flow rate of the injection fluid through the handpiece engine and tip. In one system an internal adaptive PID pressure controller loop regulates and maintains the pressure of the container.
The injection fluid flows from the container through a length of tubing to the handpiece. Inside the handpiece is a spring and ball that together construct a check valve. The check valve prevents the pressure that is built up inside the handpiece engine from moving the injection fluid back upstream. The cracking pressure of the check valve is the minimum upstream pressure at which the valve will operate, i.e., allow fluid to flow through the check valve. Knowing the cracking pressure of the check value allows the system to control the pressure of the container that supplies the injection fluid so that the optimum flow of injection fluid is provided. In a spring and ball check valve, the cracking pressure of the check valve is determined largely by the stiffness of the spring. Since the stiffness of the spring can vary from check value to check valve, there is a need for determining the pressure that provides a certain flow rate for each check valve and using the determined pressure to control the pressure of the injection fluid container during surgical operation to achieve optimum handpiece performance.